[Feat] Let users default supported communications messages to fast mode

[roomote-roomote]

​Opened on behalf of @mrubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Added the opt-in R_COMMUNICATIONS_FAST_MODE_SETTING_ENABLED deployment flag and documented it for self-hosted and production deployments.
• Added a deployment-gated Personal Settings switch backed by the generic communicationsFastModeDefault preference and communications_fast_mode_default user metadata field.
• Applied the preference to mapped Slack users in both normal and configured auto-start channels while preserving explicit !fast, fast-thread continuation, removed !eval, bot-authored auto-starts, and non-Slack routing behavior.
• Added focused environment, persistence, UI-gating, and Slack-routing coverage.

Why this change was made

Users can opt into fast mode as the default for supported communications without requiring a provider-specific preference model. The generic contract is ready for future communications providers, while this change deliberately keeps runtime behavior limited to Slack.

Impact

When the deployment flag is enabled, users can default their Slack messages to fast mode from Personal Settings, including messages posted in configured auto-start channels. The preference remains hidden, inert, and immutable while the flag is disabled. Explicit !fast still takes precedence, no Discord behavior is added, and no database migration or draft-only compatibility layer is required because the value remains in existing users.metadata JSONB.

[roomote-community]

No new code issues found. See task

• Auto-start channels bypass the Slack fast-mode default and still create standard tasks for unprefixed messages.

_{Reviewed 11af078}

[roomote-roomote]

Fixed in f809b93: opted-in human messages now enter fast mode before configured channel auto-start routing, with regression coverage. The stale capability assertions that failed Test CI were updated, and the full web test suite now passes. See task.

[roomote-roomote]

Updated in 11af078: renamed the deployment flag to R_COMMUNICATIONS_FAST_MODE_SETTING_ENABLED, the preference/capability API to communicationsFastModeDefault / communicationsFastModeDefaultAvailable, and persisted metadata to communications_fast_mode_default, with matching UI, tests, compose config, examples, and docs. Runtime reads remain confined to Slack routing; deployment gating and explicit !fast precedence are unchanged, and no Discord behavior or draft-only compatibility layer was added. Focused tests (110), full web tests (2,978), full API tests (1,748), env tests (79), lint/type checks, pre-push gates, docs validation, and knip passed. Repo-wide test retries also exposed unrelated environment/flakiness gaps: a missing GitHub App private-key test variable, transient timeout-only failures that passed in isolation, and deployment validation requiring an unavailable Docker socket. Visual proof could not reach Personal Settings because the local app remained behind the inference setup gate. See task.